Because the princesses have nothing to do with it, those worlds were chosen because they fit the fairy tale style of the game, it's in the interview. If the princesses were important in Unchained then we would know by now.
If they don't announce Enchanted Dominion during the E3 it will never come or any other world beyond that. Why? Because new world content is really the rarest of them all. If you followed the development of [chi] you would see how long it took to even get some new rooms. Beast's Castle was released over two years after the game started, that's why "finalizing the assets" is just laughable.
You're being awfully pessimistic of the progression of Unchained X. The Princess of Hearts do have significance within the lore of X: they were the hearts of pure light that restored the worlds after they all fell into Darkness. Yes, the Princesses that we currently see are not the real deal: that doesn't mean that the real Princess of Heart will not be relevant for X/Unchained X. We just have to wait for a story update.
Story content isn't the only factor of course. Chi is likely closing because it isn't making enough revenue. Unchained is probably succeeding better and it is more accessible to fans, making it foolish for them to continue to support both. Story content isn't going to be the only factor that leads to the games success, but you have to be joshin me if you think they are unrelated. When KHUx's story "ends" I can guarantee you that numbers of active players, and thus potential revenue, will see a dramatic drop.
My thoughts exactly, Square Enix and Disney have themselves a very profitable, critically acclaimed mobile app. They're not going to waste the potential of it just because X is ending.